Save article (to cache tags) before updating follow points (#15637)

Fixes a failure where the follow did not calculate implicit
points correctly.

The underlying issue was that the factory created `article` did not have
a cached_tags value containing `tag.name`
saved (`article.tags.pluck(:name)` was correct, but the cached_tags
was not). This happens automatically when saving an article from the
post editor, and any time we subsequently modify an article.

Supersedes #15456

spec/workers/follows/update_points_worker_spec.rb

@@ -20,6 +20,7 @@ RSpec.describe Follows::UpdatePointsWorker, type: :worker do
 
       user.follow(second_tag)
       user.follow(tag)
+      article.save
     end
 
     it "calculates scores", :aggregate_failures do